The Stensund Wastewater Aquaculture plant was built in 1989. The plant is contained in a greenhouse because of its northern location with a temperate climate. The plant receives wastewater from 40 persons and has a residence time of 30 days. The purification concept is to use a constructed aquatic food chain (algae, zooplankton and fish) combined with hydroponic culture of higher plants to further deplete the nutrients in the wastewater. However, the production of algae and zooplankton has not been working as expected from pilot studies. The purpose of this investigation was to study if toxic chemicals were limiting the production of zooplankton (Daphnia magna) in the aquaculture treatment system. Grab samples were taken at six sites and tested for acute toxicity to Daphnia magna and analyzed for BOD, COD, ammonium-, nitrite-, nitrate- and total nitrogen, total phosphorus and heavy metals (Cd, Cr, Cu, Ni, Pb, Zn). Toxicity to D. magna was also determined after addition of EDTA, sodium thiosulphate and pH adjustment of the water to 6, 7 and 8, in order to assess if heavy metals and/or pH-dependent toxicants were present. Potential toxicants, with concentrations high enough to intoxicate Daphnia magna, were Cu, ammonia and nitrite. Subsequent improvement of nitrification reduced the ammonia and nitrite concentrations as well as the toxicity. Furthermore, the improved production of algae and Daphnia at the Stensund Wastewater Aquaculture plant verified that the primary cause of toxicity was ammonia.

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